CVE-2026-34230: Rack has quadratic complexity in Rack::Utils.select_best_encoding via wildcard Accept-Encoding header

April 2, 2026 (updated May 13, 2026)

Rack::Utils.select_best_encoding processes Accept-Encoding values with quadratic time complexity when the header contains many wildcard (*) entries. Because this method is used by Rack::Deflater to choose a response encoding, an unauthenticated attacker can send a single request with a crafted Accept-Encoding header and cause disproportionate CPU consumption on the compression middleware path.

This results in a denial of service condition for applications using Rack::Deflater.

References

Code Behaviors & Features

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